Filters formed from resonators are widely used in data transmission and in particular, telecommunications, for example in base stations, radar systems, amplifier linearization systems, point-to-point radio, and RF signal cancellation systems. Although a specific filter is chosen or designed dependent on the particular application, there are certain desirable characteristics that are common to all filter realisations. For example, the amount of insertion loss in the pass-band of the filter should be as low as possible, while the attenuation in the stop-band should be as high as possible. Further, in some applications the frequency separation between the pass-band and stop-band (guard band) needs to be very small, which requires filters of high order to be deployed in order to achieve this requirement. However, the requirement for a high order filter is always followed by an increase in the cost (due to the greater number of components that such a filter requires) and space.
One of the challenging tasks in filter design is to reduce their size while retaining much of their electrical performance such that they are comparable with larger structures. One of the main parameters governing filter's selectivity and insertion loss is the so-called quality factor of the elements comprising the filter—the “Q factor”. The Q factor is defined as the ratio of energy stored in the element to the time-averaged power loss. For lumped elements that are used especially at low RF frequencies for filter design Q can be of the order ˜60-100, whereas for cavity type resonators Q can be as high as several 1000s. Although lumped components offer significant miniaturization their low Q factor prohibits their use in highly demanding applications where high rejection and/or selectivity is required. On the other hand cavity resonators offer sufficient Q but their size prevents their use in many applications.
With the advent of small cells where the footprint of the base-station should be low the problem of reducing the size of such filters is becoming more acute. This is also the case in the currently observed trend of macro cell base-stations which seek to provide multiband solutions within a similar footprint to that of single band solutions without sacrificing system's performance. It would be desirable to reduce a resonators size while maintaining many of its properties.